Audio amplifiers of progressively higher power outputs have been constructed by audio engineers in an attempt to achieve improved fidelity and lower distortion levels. This has necessitated the use of progressively more and larger, and therewith more expensive power semiconductors. Power losses also increase with increasing power outputs, and in order to prevent the power semiconductors being destroyed by high temperatures, it is necessary to provide cooling means of very high capacity, which increases both the cost and the volume of the amplifier.
In the case of conventional amplifiers, particularly so-called push-pull amplifiers, the linear power amplifier is driven by two direct current sources of opposite polarity in relation to earth or to a reference. The signal to be amplified shall swing between the supply voltages--up and down--in relation to earth or the reference. As a result, current will flow through the amplifying element/transistor at the same time as a part of or the whole of the supply voltage lies across the transistor. (The whole of the supply voltage lies across the amplifying element at the same time as current flows to the load/loudspeaker when the load is reactive. Commercially available loudspeakers have a relatively large reactive impedance over a major part of their frequency ranges.) This results in a significant loss in power and therewith relatively low efficiency.
The ability of the amplifying element to deliver high reactive currents when the whole of the supply voltage is applied across said element also places high demands on power endurance and large so-called "Safe Operation Area". This requires a large number of power transistors to be connected in parallel at high power outputs, which greatly increases the cost of the amplifier.
According to one known method of minimizing power losses, the audio signal modulates the pulse width of a square wave whose frequency is much higher than the frequency of the audio signal. When the pulse width molulated square wave switches the power amplifier between the supply voltages such that current will only flow when the amplifying elements/power transistor are fully conductive and no voltage lies across said elements, only a small power loss is caused by non-ideal components. The audio signal is then recovered in a lowpass filter succeeding the amplifier, which filters-off the high frequency square wave.
Since the amplifying elements/power transistors are switched on-and-off, the efficiency or power endurance is not influenced to any great extent when the load/loudspeakers require reactive currents. This ability and the small power losses obtained are two of the important advantages obtained with class D-amplifiers.
It is thus the pulse width of the high-frequency square wave which corresponds to the instantaneous value of the audio signal. When the height of the square-wave amplitude is influenced, the audio signal is multiplied by this influence, since both pulse-width modulation and amplitude influence are recovered downstream of the lowpass filter. This places high demands on supply voltage stability and the suppression of supply voltage disturbances in class D-amplifiers. If the regulation to supply voltage is to be effected sufficiently well to achieve low amplitude modulation distortion, the efficiency of the network part is impaired, so that power losses are transferred from the power amplifier to the regulated current supply.
A known method of reducing amplitude modulation distortion involves the introduction of negative feedback. The degree of negative feedback, however, is restricted due to the phase shift in the output filter, and because it is not possible at present to give the square wave in practice a frequency which is sufficiently high (due to delay in the transistors) to eliminate the phase effect of the lowpass filter in that frequency range required to feed back the audio signal.
Another problem associated with class D-amplifiers is that when the polarity of the square wave changes, the energy in the lowpass filter must be handled via switching diodes which feed the energy back to the supply voltages. If the drop in voltage across the power semiconductors and across the so-called recovery diodes does not coincide, amplitude modulating distortion again occurs on the audio signals. Solutions to this problem are known (Attwood, U.S. Pat. No. 4,182,991), although these solutions increase the complexity.
A further problem with class D-amplifiers resides in the matching of switching times between the positive and the negative amplifying elements. Incomplete matching results in power losses, durability problems and crossover distortion. Solutions to this problem are also known (Attwood, U.S. Pat. No. 4,182,922), although these solutions reduce the reproducibility of the amplifier, because the apparatus according to this solution requires trimming.
Another known method of reducing power losses in conventional push-pull amplifiers involves controlling the direct current sources in a manner such that the supply voltages will also be influenced by the audio signal, either continuously or stepwise. This reduces the voltage across the amplifying element at the same time as current flows through said element, resulting in a relatively small power loss.
The U.S. patents to Waehner, Sampei and Carver (U.S. Pat. No. 3,772,606, 3,961,280 and 4,484,150 respectively) describe audio amplification with stepped supply voltages. The delay times of commercially available power semiconductor are, in practice, too long in order for switching to take place in a distortion-free fashion, particularly in the uppermost octave of the audio frequency range.
In the U.S. patents to Jensen and Hamada (U.S. Pat. No. 3,426,290 and U.S. Pat. No. 4,054,843 respectively), there are described audio amplifiers with continuous, adaptive supply voltages, where the supply voltage unit is effected through the medium of controlled pulse-width modulated means. Jensen describes a so-called "single ended-amplifier", i.e. an amplifier with a single supply voltage, whereas Hamada describes both a single ended and a push-pull audio amplifier with controlled pulse-width modulated supply voltage. Neither the stepped nor the continuously controlled supply voltage principles, however, give a solution to the problem of how the audio amplifier shall be able to deliver reactive current in the absence of significant power losses. At least one supply voltage, in the case of push-pull amplifiers, will lie across the amplifying element at the same time as a full reactive current flows through said element. Similar to the case with the Hamada patent, this is because the supply voltage can only be adjusted down to a minimum around the reference point/earth.